University of Virginia
Physics Department

## Atomic Mass

A Physical Science Activity

### Student Activity

Materials

• Marbles (fairly heavy) of two different colors (to represent protons and neutrons)
• Small (very light) beads (to represent electrons)
(These are just suggestions for materials - any suitable substitutions will work. You just need to make sure that the protons and neutrons have approximately the same mass and that the electrons have a much smaller mass.)
• Balance
• Container that can be sealed and covered (some options are a baby food jar, racquetball container, tennis ball container, etc.)

Procedure

1. Mass each particle and record that information in the data table. (Depending upon the precision of your balance and the weight of the objects you're using, it may be necessary to take the mass of more than one particle in order to obtain a reading on the balance. The accuracy of your numbers will also improve if you follow this procedure. For example, you may want to take the mass of 50 "electrons" and divide that number by 50 to calculate the mass of one of those "electrons".)
2. Take the mass of the "atom" that your teacher gives you. Subtract the mass of the empty container so that you only get the mass of the particles inside.
3. Open up the atom and count the number of each particle and record that in the data table. Multiply the number of each particle by the mass of one of those particles and record that answer in the data table. Add all of the particles' masses to find a total mass of the atom and compare that to the measured mass.
4. Compare the number of protons to the number of electrons to other groups' results.

Data Sheet

Data Table:

 Mass of one particle "protons" "neutrons" "electrons"

Atom 1:

 Mass of atom + container = ______________ g Mass of empty container = ______________ g Mass of atom = ______________ g
 "protons" "neutrons" "electrons" -------------- # of particles -------------- Total mass from particle Total Mass

Atom 2:

 Mass of atom + container = ______________ g Mass of empty container = ______________ g Mass of atom = ______________ g
 "protons" "neutrons" "electrons" -------------- # of particles -------------- Total mass from particle Total Mass